There are cases when PluggableListMorph has is inner list cache in nil:

getListElementSelector: aSymbol
�� "specify a selector that can be used to obtain a single element in the underlying list"
�� getListElementSelector := aSymbol.
�� list := nil.� "this cache will not be updated if getListElementSelector has been specified, so go ahead and remove it"
...

But, there are some methods that send messages to that nil list and blow up :D, such as:

backgroundColorFor: aRow

�� | anItem return |
�� aRow ifNil: [ ^nil ].
�� anItem := list at: aRow ifAbsent: [ ^ nil ].
...

Now, two solutions comes to my mind:

put one more ward in:

backgroundColorFor: aRow

�� | anItem return |
�� (aRow isNil or: [ list isNil ]) ifTrue: [ ^nil ].
�� anItem := list at: aRow ifAbsent: [ ^ nil ].
...

Or use #getList, which seems to be the most... cleaner.

backgroundColorFor: aRow

�� | anItem return |
�� aRow ifNil: [ ^nil ].
�� anItem := self getList at: aRow ifAbsent: [ ^ nil ].
...

?

Guille,
(meanwhile, opening a ticket :)).